Double-exchange-driven spin pairing at the (001) surface of manganites

Study of the (001) surface of the La_1-xCaₓMnO₃ system in various magnetic orderings points to a general occurrence: z² dangling bond charge---which is ``invisible'' in the formal valence picture---is promoted to the bulk gap and/or Fermi level region. This unexpected occurrence, obtained from first-principles calculations, drives a double-exchange-like process that serves to align the surface Mn spin with its subsurface neighbor, regardless of the bulk magnetic order. For heavy doping, the locally ``ferromagnetic'' coupling is very strong and the moment enhanced by as much as 30% over the bulk value.